This invention pertains to an ink set for inkjet printing, in particular to an ink set comprising at least one nonaqueous ink and at least one aqueous fixer fluid. The invention also pertains to a method of inkjet printing with this ink set.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor.
Ink can comprise colorant that is dissolved or dispersed in the ink vehicle. The ink vehicle can be aqueous or nonaqueous and the ink is referred to as aqueous or nonaqueous ink, accordingly.
Aqueous ink is advantageous because water is especially environmentally friendly. However, water is relatively volatile and aqueous-based inks can dry out quickly on the printhead. Humectants are often added to aqueous ink to retard drying, but precipitation of formulation ingredients can occur as water is lost even if the ink is not “dry”. Also, images printed with colorants that are dispersible/soluble in water tend to be prone to rub-off and smear. Nonaqueous inkjet inks can be formulated with solvents having low volatility (i.e. slow drying or non-drying) and can employ colorants that provide highly rub- and smear-resist images. When jetted on paper though, nonaqueous inks tend to penetrate deeply, similar to penetrating aqueous inks, causing loss of color density, blurring and strikethrough.
Penetrating aqueous inks are formulated with a vehicle that penetrates rapidly into the printed substrate, and in so doing becomes quickly dry to the touch and therefor appears “fast drying”. However, penetrating formulations also tend to move outward as well as downward which results, as was already mentioned, in blurring and strikethrough.
One way to reduce blurring and strikethrough in fast drying aqueous inks is to apply a fixing solution to the media prior to printing the ink. See, for example, U.S. Pat. Nos. 5,746,818, 6,450,632, US20020044185 and EP1258510, and commonly owned U.S. application Ser. No. 10/755,630 (filed Jan. 12, 2004, claiming priority from U.S. Provisional Application Ser. Nos. 60/440,493 (filed Jan. 16, 2003) and 60/449,760 (filed Feb. 25, 2003)), the disclosures of which are incorporated by reference herein for all purposes as if fully set forth.
Fixers commonly operate by creating an adverse charge-charge interaction with the colorant in the aqueous ink thereby precipitating and fixing the colorant on the substrate surface. This mechanism is effective with aqueous inks because the colorant is typically stabilized to dispersion or solution by an ionic mechanism. A fixer with opposite charge effectively destabilizes and fixes the colorant. However, these methods of fixation are not expected to be operable with nonaqueous inks because charge stabilization is not in effect in a low dielectric constant (nonaqueous) vehicle.
A need thus exists for fixer fluids that can improve the image quality of nonaqueous inks.